Iran has denied involvement in a drone attack on a US base near Jordan’s border with Syria that killed three US troops.
The US blamed the attack, which also left dozens injured, on “radical Iranian-backed militant groups”.
US President Biden vowed revenge and said: “We shall respond.”
It is the first time that a strike has killed US troops in the region since the start of the war in Gaza triggered by Hamas’s 7 October attack on Israel.
There have been other attacks on US bases in the region, but before Sunday there were no fatalities, according to the US military.
The Islamic Resistance in Iraq has claimed it was behind the attack.
The umbrella group emerged in late 2023 and is comprised of several Iran-affiliated militias operating in Iraq. It has claimed other attacks against US forces in recent weeks.
In a statement, the group said it had targeted three US bases in Syria and Jordan – Shaddadi, Rukban and Tanf, as well as an Israeli oil facility in the Mediterranean.
The Iraqi government has condemned the attack and called for “an end to the cycle of violence” in the Middle East.
Iraqi government spokesman Bassem al-Awadi said it was willing “to collaborate on establishing fundamental rules to prevent further repercussions in the region and curb the escalation of conflict”.
Mr Biden said the US “will hold all those responsible to account at a time and in a manner of our choosing”.
Iran denied US and British accusations that it supported militant groups blamed for the strike.
Nasser Kanaani, Iran’s foreign ministry spokesman, said it was “not involved in the decision making of resistance groups” in how they chose to “defend Palestinians or their own countries”.
The White House said Mr Biden was briefed about the attack on Sunday morning by US Defence Secretary Lloyd Austin and other officials.
“Jill and I join the families and friends of our fallen – and Americans across the country – in grieving the loss of these warriors in this despicable and wholly unjust attack,” Mr Biden said in a statement.
The names of the servicemen killed and injured have not yet been released as officials work to notify their families.
On a visit to South Carolina, Mr Biden said “we had a tough day last night in the Middle East. We lost three brave souls”.
UK Prime Minister Rishi Sunak urged Iran to “continue to de-escalate tensions” in the region. He said: “We absolutely condemn what has happened over the past couple of days.”
“We stand resolutely with our allies to bring stability and peace to the region,” Mr Sunak said.
US officials said at least 34 military personnel were being evaluated for possible traumatic brain injury, and that some of the injured soldiers were medically evacuated from the base for further treatment.
They also said the drone struck living quarters at the base, which, if confirmed, could explain the high number of casualties.
US Central Command and President Biden said the attack was on a base in Rukban, northeastern Jordan, near the Syrian border. It was later named by US officials as Tower 22.
In December, US officials said that US bases in Iraq and Syria had been attacked at least 97 times since 17 October.
Last month, the US carried out airstrikes against Iran-affiliated groups after three US servicemembers were injured, one critically, in a drone attack on a base in northern Iraq.
Earlier in January, one retaliatory US strike in Baghdad killed a militia leader accused of being behind attacks on US personnel.
In a pre-recorded interview with ABC News that aired on Sunday morning, Chairman of the Joint Chiefs of Staff Gen CQ Brown said that America’s aim in the region is to “not have the conflict broaden”.
“The goal is to deter them and we don’t want to go down a path of greater escalation that drives to a much broader conflict within the region.”
US and coalition troops are also stationed in the Red Sea after the Iran-backed Houthis began attacking commercial ships in the region. The Yemen-based group says it is targeting vessels in the region in support of Palestinians in Gaza, where Israel is fighting Hamas.
The US military has previously said “these unlawful actions have nothing to do with the conflict in Gaza”.
“The Houthis have fired indiscriminately into the Red Sea, targeting vessels impacting over 40 countries around the world,” US Central Command has said.
Two Navy Seals are presumed dead after they went missing in January during an operation off the coast of Somalia to seize Iranian-made weapons bound for Houthis in Yemen.